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Epitaxial ZnO and Al-doped ZnO (AZO) thin films were grown on (0001)-sapphire substrates using pulsed laser deposition. The photoluminescence spectrum of the highly conductive (1.3 × 103 S cm−1), as-grown AZO shows a poor near band edge (NBE) emission (3.30 eV) and no deep level emission at room temperature. In addition, the peak (3.386 eV) for the free excitons of AZO showed thermal quenching behavior with two activation energies (38.2 and 10.0 meV). The poor NBE emission is attributed to the nonradiative recombination center created by Al doping. Highly conductive (6.0 × 102 S cm−1) and intense NBE emitting AZO films could be achieved by the reduction of the nonradiative recombination centers through hydrogen annealing.

Photoluminescence measurements on individual CdSe / ZnSe / ZnMnSe quantum dots in a magnetic field up to 11 T both parallel and perpendicular to the sample growth plane at 1.6 K reveal a qualitative difference between samples with different strength of the sp-d exchange interaction controlled by means of varying the thickness of the nonmagnetic ZnSe layer. The observed difference is related with the dependence of the non-radiative Mn assisted recombination and the spin relaxation on magnetic field and the strength of the exchange interaction.

We study the effect of a red shift and a considerable enhancement of photoluminescence (PL) intensity from a freshly etched porous Si (PS) thin film after prolonged laser irradiation or after aging in atmosphere. Both effects coincide with the appearance of Si–OH and Si–O–Si vibration modes in the Fourier transform infrared (FTIR) absorption spectra. The red shift is attributed to a pinning of the band gap of the light-emitting Si nanocrystals (NCs) due to the formation of Si–OH and Si–O–Si bonds. Using theoretical calculations, we estimated the electron and hole energy shifts caused by the interaction of the electronic states of the Si NCs with the surface vibrations, and correlated the observed PL enhancement with resonant coupling between the quantized valence sub-levels in the Si NCs and surface vibration modes.

In spite of huge progress in improving the internal quantum efficiency of organic light-emitting diodes (OLEDs), these devices still suffer from poor light out-coupling. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmons at metallic electrodes. Their relative strength and the mutual dependence on the OLED structure have been studied both experimentally and by numerical simulation. Here, we consider the impact of the radiative quantum efficiency of the emitter material on predictions of light extraction from OLEDs. Competing processes resulting in non-radiative recombination of charge carriers usually reduce the emitter quantum efficiency in a real device. We show that optical simulation leads to erroneous conclusions when neglecting these competing processes. Furthermore, we demonstrate a method, which allows determining both the radiative quantum efficiency and the charge recombination factor via simulation based analysis of experimental data. This analysis of device efficiency is applied on a set of red-emitting electrophosphorescent devices.

Complex investigations, including measurements of C – V curves, studies of I – V curves in the 10 mV–5 V voltage and 10−14–1 A current ranges and QE(I) dependences, were carried out on the set of InGaN/GaN LEDs with different active region designs, and with different maximum external quantum efficiency (QE), changing from 16% to 40%. Common regularities of carriers transport for all investigated LEDs were found. They were independent of barriers height and width, number of wells and nanostructural arrangement, and can be related to the non-radiative carrier recombination dynamics taking place in the presence of the extended defect system piercing the LED active region. The results obtained allow us to assume that the effect of the extended defect system on QE, is indirect and consists mainly of the electric field redistribution in the active region, as the properties of shunts formed by these defects are changed under the current injection. The point defect effect is the most probably seen in the narrow current range corresponding to the QE maximum. It was assumed that the Auger processes involving recombination centers, localized on the extended defects, could be the cause of quantum efficiency decay at current densities larger than 50 A/cm2.

Experimental evidences are presented for the first time that singlet oxygen generated ex situ acts as an inhibitor of nonradiative recombination in porous silicon (PSi). This effect is observed on a pristine PSi as well as on degraded porous layers quenched by ozone adsorption. A photoluminescence (PL) enhancement produced by singlet oxygen is accompanied with only slight oxidation of a PSi. We assume that the observed effect on PL efficiency is due to gentle selective oxidation of single defects on silicon nanocrystal surface.

Experimental results concerning the steady-state photoluminescence (PL) studies in n and p modulation doped and undoped GaInNAs/GaAs quantum wells are presented. The effects of modulation, type of doping and nitrogen concentration on the PL and the temperature dependence of the band gap, carrier localization and non-radiative recombination are investigated. Increasing the nitrogen composition decreases energy band gap as expected. The n-type modulation doping eliminates most of the defect-related effects and blue shifts the energy band gap. However, the p-type doping gives rise to additional features in the PL spectra and red shifts energy band gap further compared to the n-type-doped material.

Self-assembled InAs quantum dots (QDs) on GaAs(0 0 1) substrate were grown by molecular beam epitaxy (MBE) at a growth temperature of 490 °C. Two different families of dots were observed in the atomic force microscopy (AFM) image and ambiguously identified in the photoluminescence (PL) spectra. Temperature-dependent PL study was carried out in the 8–270 K temperature range. The integrated-PL intensity behavior of the two QDs populations was fit with the help of a rate equations model. It is found that the evolutions of the integrated-PL intensity of the two QDs population were governed by two regimes. The first one occurs in the 8–210 K temperature range and reveals an unusual enhancement of the integrated-PL intensity of the larger QDs (LQDs) class. This was attributed to the carrier supplies from the smaller QDs (SQDs) class via the tunneling process. The second one occurs in the 210–270 K temperature range and shows a common quench of the PL signals of the two QDs families, reflecting the same thermal escape mechanism of carriers.

Temperature characteristics and thermal resistance of InGaAs/GaAs laser diode (LD) is investigated from below to beyond the lasing threshold. Spectrally-resolved emission measurements show that the heat generated in the active region is induced by the radiative energy transfer of free carriers. Below the lasing threshold, nonradiative recombination induces large heat generation in the active region. Beyond the lasing threshold, Joules heating due to the series resistance of the LD dominates the heating response. The dependence of the associated thermal resistance on different bonding configurations and its correlation with the output power is also discussed. Epi-side up and epi-side down bonding of LDs reduces the device temperature by ~ 30% and ~ 50%, respectively.

We studied the nonthermal mechanism of quantum efficiency (QE) roll-off in InGaN-based multiple-quantum-well LEDs on sapphire and bulk GaN substrates with In content ranging from 1% to 28%. Both the efficiency evolution and peak energy shift appear to be strongly dependent on the In content, but nearly independent of the dislocation density in the active region. The QE of the green LED peaks at a current density as low as 1.4 A/cm 2 , and decreases dramatically as current is increased, whereas the UV LED has a nearly constant QE at currents up to 1 kA/cm 2 . In contrast to minimal current-induced peak shift and spectral broadening in the near-UV and UV LED, the green LED has a monotonic peak blueshift of a total amount of ∼ 110 meV and a spectral broadening by a factor of 2. These results lead to a conclusion that carrier delocalization followed by nonradiative recombination at misfit defects is the major nonthermal mechanism of the efficiency roll-off in InGaN-based visible LEDs.

We studied temperature-dependence of electroluminescence (EL) in fluorescent dye-doped and undoped organic light-emitting devices. Results show that the EL efficiency of the fluorescent dye-doped devices increases substantially with rising temperatures, as opposed to the dramatic decrease in EL efficiency of the undoped devices. Our experimental results demonstrate directly that thermally activated exciton diffusion is responsible for the reduced luminescence efficiency of the undoped devices at higher temperatures. However, in the doped devices, EL spectra show that excitons are effectively localized onto the guest molecules, which prevents them from being quenched by randomly distributed nonradiative recombination sites and from exciton–exciton annihilations during thermally activated diffusion, resulting in an enhanced EL efficiency at elevated temperatures.

Electroluminescence in the range of 6–12 μm is observed from an Sb-based type-II interband quantum cascade structure. The LED structure has 30 active/injection periods. We have studied both top-emitting and flip-chip mount bottom emitting LED devices. For room temperature operation, an increase, saturation and decrease in light output occur at successively higher injection currents. An increase of about 10 times in light output occurs when device is operated at 77 K compared to room temperature operation. This increase is attributed to reduced Auger non-radiative recombination at lower temperatures. The peak-emission wavelengths at room temperature and 80 K operation are 7 and 10 μm, respectively. These devices can be used for high-temperature simulation in an infrared scene generation experiment.

Photoluminescence (PL) and Raman spectra of Ge/Si quantum dot (QD) structures grown using conventional molecular beam epitaxy, MBE, (sample M) and pulsed ion beam-assisted MBE (sample MI) were measured before and after vacuum annealing and hydrogen passivation. Two broad bands are observed in the PL spectra, one ascribed to the QDs and another one to radiation defects. Sample MI exhibits a higher overall PL intensity. The post-growth vacuum annealing partially suppresses the defect-related emission, enhances the QD-related PL in sample M, but almost does not affect the latter emission in sample MI. After the atomic hydrogen treatment that passivates the defect-related radiative and nonradiative recombination centers, a high-energy shift by 20–25 meV of the QD PL peak in sample MI appears when compared to sample M. The analysis shows that the shift is mainly due to a reduction of the QD size, which occurs in the case of the ion-assisted MBE. We conclude that the radiation defects created at the QD growth temperatures do not introduce nonradiative recombination channels, on the contrary to the defects caused by the nonequilibrium character of the MBE growth. In situ high-temperature ion bombardment during the QD growth pushes the epitaxial layer system towards equilibrium, thus enhancing the QD PL intensity.

In narrow-gap semiconductors, non-radiative Auger recombination losses (via conduction, heavy-hole, spin split-off hole, heavy-hole processes) are sensitive to how the band gap energy, E 0 , compares to the spin-orbit splitting, ∇ 0 . However, narrow-gap band structures have often not been very well characterised. We report on the mid-infrared photo-modulated reflectance of E 0 and E 0 + ∇ 0 transitions in high-quality, InAs-rich InAsSb and GaInAsPSb samples as functions of both temperature (T= 10-300 K) and antimony content ≤ 22.5%, for wavelengths up to ∼ 4.75 μm. The measured T-dependence of E 0 is generally consistent with that accepted in the literature and we confirm that Δ 0 is T-independent. As a function of Sb fraction, E 0 is consistent with the positive bowing parameter of +670 meV quoted in the literature. However, Δ 0 does not exhibit the currently accepted positive bowing parameter of + 1170 meV: rather, a best fit to our data tentatively suggests a negative bowing of ∼- 165 meV. Due to the importance of Δ 0 in predicting and interpreting the performance of InAsSb-based devices, this result is expected to have a significant impact.

Ionoluminescence (IL) of kyanite single crystals during 100 MeV Si8+ ion irradiation has been studied in the fluence range 1.87–7.50×1011 ions/cm2. Photoluminescence (PL) of similar dimensional crystals was recorded with same ions and energy in the fluence range 1×1011–5×1013 ions/cm2 with an excitation of 442 nm He–Cd laser beam. A sharp IL and broad PL peaks at ∼689 and 706 nm were recorded. This is attributed to luminescence centers activated by Fe2+ and Fe3+ ions. It is observed that up to a given fluence, the IL and PL peak intensities increase with increase of Si8+ ion fluence. The stability of the chemical species was studied on with and without irradiated samples by means of FT-IR spectroscopy. The results confirm that the O–Si–H type bonds covering on the surface of the sample. This layer might be acting as a protective layer and thereby reducing the number of non-radiative recombination centers.

An overview of recognized structural defects, impurities and related trapping levels and their role in the scintillation mechanism is provided and discussed in single crystal materials belonging to tungstate, Ce- or Pr-doped aluminum perovskite, garnet and finally to Ce-doped silicate scintillators. New achievements and open problems in deeper understanding of electron and hole self-trapping phenomena and of the nature of defects in the crystal structure and their ability to localize migrating charge carriers are indicated. Fast optical ceramics and nanocomposite materials are pointed out as possible future advanced scintillators. Such novel technologies can in principle explore materials which are not available in the bulk single crystal form, but their figure-of-merit is dramatically dependent on the surface-interface defect states and related trapping and nonradiative recombination phenomena. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Twenty five years ago Arakawa suggested that by confining carriers in three dimensions (in quantum dots) a temperature insensitive threshold current (Ith) could be achieved in semiconductor lasers. In this paper we discuss investigations on state-of-the-art 1.3 μm InAs/GaAs undoped and p-doped quantum dot lasers for telecommunication applications and discuss the extent to which this original hypothesis has been verified. In this study, the threshold current and its radiative component (Irad) are measured as a function of temperature and pressure. The results show that although the radiative component of the threshold current can be temperature insensitive in undoped quantum dot lasers, a strong contribution from non-radiative Auger recombination makes the threshold current highly temperature sensitive. We find that p-doped devices can have a temperature insensitive Ith over a limited range around room temperature resulting from an interplay between an increasing non-radiative Auger current and decreasing radiative current. The decrease in Irad, also observed below 200 K in undoped devices, is attributed to an improvement in the carrier transport with increasing temperature. Gain measurements show that even if p-doping is successful in reducing the effect of gain saturation, the modal net gain of p-doped devices is less than in undoped lasers due to increased non-radiative recombination and non-thermal carrier distribution.

The aging behavior of red-orange AlGaInP light-emitting diodes (LEDs) is investigated. It is found that the amount of magnesium and tellurium doping as well as of the oxygen incorporation influence the device degradation. The generation of non-radiative recombination centers in the active layer is responsible for the decrease of the LED light output during operation. The kinetics of the light aging as well as the configuration of the non-radiative recombination centers emerging during degradation depend on the doping species. High stress temperatures can accelerate the device degradation. For the aging of Te-rich device, the activation energy of 660 meV for the degradation process is determined. Deep level transient spectroscopy is used to investigate the properties of the non-radiative recombination centers. Four deep traps are detected in aged devices. One trap with activation energy of about 1 eV is found in all aged devices. In the aged O-rich device, one additional deep trap is observed with slightly lower activation energy. In the aged Te-rich device two additional shallower traps emerge during degradation.

In order to observe the effect of intra-band gap excitation on the photoluminescence (PL) properties of undoped InP and iron doped InP (InP:Fe), PL measurements were performed in InP crystals with thickness of 360 μm and area of about 4×3 mm2, grown by the liquid encapsulated Czochralski (LEC) technique upon excitation with both Ar-ion laser and 980 nm light. The PL intensities for InP:Fe under 980 nm wavelength light illumination relative to no illumination increased by about 52%, 33%, and 12% for the 1.337, 1.380, and 1.416 eV peaks, respectively, at 10 K, whereas there was no illumination effect for undoped InP. This is a strong indication that Fe centers play a role as non-radiative recombination centers to decrease the PL intensity. PL experiments were performed in the spectral range of 1320–1440 meV for InP in the sample temperature range of 10–160 K. The electron and hole photoionization cross-sections at 980 nm wavelength light illumination were calculated as σ n 0 [ Fe 2 + / 3 + → CB ] = 2.34 × 1 0 - 18 cm - 2 and σ p o [ VB → Fe 2 + / 3 + ] = 8.20 × 1 0 - 17 cm 2 , respectively.

The correlation between threading dislocations (TDs) and nonradiative recombination centers in InN films was investigated by infrared cathodoluminescence (CL). Samples were grown on nitridated (0001) sapphire substrates with a low-temperature-grown InN buffer layer by radio frequency molecular-beam epitaxy (RF-MBE). Panchromatic CL images of the InN films showed a high density of dark spots in a range of 108 cm−2 to 109 cm−2. The sample with a higher density of TDs had a higher density of CL dark spots. A depth-dependent CL measurement confirmed that CL dark spots aligned almost vertically in the film like TDs. Reasonable correlation between TDs and the nonradiative regions was also observed by a cross-sectional CL image of the InN film regrown on a microfaceted InN template, in which the TD density was dramatically reduced in part. These results suggest that threading dislocations act as nonradiative recombination centers in InN.

In this paper we will review our recent results from optical characterization studies of GaInNP. We will show that N incorporation in these alloys affects their structural and defect properties, as well as the electronic structure. The main structural changes include (i) increasing carrier localization due to strong compositional fluctuations, which is typical for all dilute nitrides, and (ii) N-induced long range ordering effects, specific for GaInNP. The observed degradation of radiative efficiency of the alloys upon increasing N content is attributed to formation of several defects acting as centres of efficient non-radiative recombination. One of the defects is identified as a complex involving a Ga interstitial atom. N incorporation is also found to change the band line up from the type I in the GaInP/GaAs structures to the type II in the GaInNP/GaAs heterojunctions with [N]>0.5%. For the range of N compositions studied ([N]≤2%), a conduction band offset at the GaInNP/GaAs interface is found to nearly linearly depend on [N] at 0.10 eV/%, whereas the valence band offset remains unaffected.

Epitaxial lateral overgrowth (ELOG) of a -plane GaN on r -plane sapphire with stripe orientations parallel to [0 0 0 1], [ 0 1 1 1 ], and [ 0 1 1 0 ] has been investigated. The emission from ELOG stripes with different orientations and from different growth facets has been compared by spectrally and spatially resolved cathodoluminescence and by temperature-dependent photoluminescence measurements. A reduction in the nonradiative recombination rate by the ELOG process is clearly observed. For all stripe orientations the ELOG wing regions exhibit increased radiative emission intensities compared to the window areas, indicating a reduced dislocation density in the overgrown areas. The highest increase of the near band edge (NBE) emission is observed in the wing regions for stripes parallel to [ 0 1 1 0 ]. In this region, the density of basal plane stacking faults (BSFs) is also reduced. NBE emission is detected up to temperatures of 250 K.

Four sizes of water-soluble thiol-capped CdTe quantum dots (QDs) have been synthesized and used to investigate the photoluminescence (PL) responses to Ag+ ions. For small particles, the CdTe QDs exhibit PL enhancement in the presence of lower concentration of Ag+ but show obvious quenching with the further increase of Ag+; for larger particles, however, PL of CdTe QDs is quenched all the time with the Ag+ addition, no PL enhancement is observed. Mechanism study shows that small QDs with more traps on the particle surface are effectively passivated by initial adsorbed Ag+, which accounts for the PL enhancement observed; after the initial traps are saturated, the excess Ag+ facilitates nonradiative recombination, resulting in PL quenching. For larger particles, the nonradiative recombination dominates the whole process even for the lower concentration of Ag+, due to the fewer traps on the QD surface. Compared with larger particles, the small CdTe QDs are more suitable for sensing Ag+ because of the more sensitive and selective PL response. To our best knowledge, this is the first systematical study on the interaction of Ag+ with different-sized CdTe QDs.

Photoluminescence (PL) of Si nanocrystals with annealing in CO2 was studied using the continuous and time-resolved PL measurements. The PL intensity enhances as annealing temperature increases, which is attributed to passivation of the nonradiative recombination centers. As the annealing temperature is 950 °C the PL intensity has a maximum value, which increases about a factor of 3 than that of the untreated sample. Based on the emission energy dependence of the PL decay time, the depth of carrier localization is found to increase after the annealing in CO2. We suggest that the oxygen atoms desorbed from CO2 diffuse to react with Si nanocrystals and introduce localized states at the Si/SiO2 interface.

Dislocation-related photoluminescence (PL) from oxygen containing Czochralski silicon (Cz-Si) processed at up to 1,400 K (HT) under atmospheric and enhanced Ar pressures (HP, up to 1.2 GPa) has been investigated. Appropriate processing of Cz-Si results in a creation of oxygen precipitates and of related defects, e.g., dislocations. Depending on processing conditions, the dislocation-related PL lines at about 0.81 and 0.87 eV are of the highest intensity while PL peaks also at 0.84 and 0.94 eV. The subsequent treatments of processed Cz-Si under pulse HP up to 2 GPa at room temperature affect PL intensity, also because of annihilation of non-radiative recombination centers formed at HT–(HP). Specific HT–HP treatments, influencing a creation of oxygen precipitates and a creation/annihilation of non-radiative recombination centers, make possible to tailor PL from Cz-Si.

Vickers indentations of ZnO crystals grown by the hydrothermal method were studied by cathodoluminescence. The defects induced by indentation influenced the luminescence spectrum, indicating the generation of non radiative recombination centers and a band close to the first phonon replica of the free exciton, in the surrounding area near the indentation. The possible nature of the defects responsible for such band is discussed. A comparison with polishing induced damage is also presented.

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.

The physics of charge-transfer processes in semiconductors is a challenging and longstanding problem. Focusing on Cr-substituted ZnSe semiconductors, important for optoelectronic and spintronic devices, several processes and their energetics are analysed using first-principles. In contrast to the properties exhibited by deep gap levels, our results for highly Cr doped ZnSe show small variations in the equilibrium configurations, forces and electronic density around the Cr for different charge states. Therefore, the delocalization of the electronic charge between the impurity and host leads to a decrease of the effective Coulomb repulsion and becomes then the fundamental mechanism to inhibit nonradiative recombination via multiphonon emission for the modes studied.

Nitrogen flow rate dependence of InGaN growth mechanism on sapphire (0 0 0 1) was studied. The indium content of the InGaN layer can be controlled by changing the nitrogen supply rate by structural and optical investigations. In case of lower nitrogen flow rate, the formation of phase separation due to dissociation of InN and condensation of indium on the growing surface were observed. In case of higher nitrogen flow rate (under N-rich condition), on the other hand, the indium content became uniform and the photoluminescence (PL) emission showed single peak. It is considered that the causes of phase separation formation were suppressed and nonequilibrium condition was enhanced by the increase of the nitrogen supply rate and the growth rate. Judging from the estimated thermal activation energies and degree of fluctuation in the indium content by PL line-width, however, the non-radiative recombination centers within the spontaneously formed potential wells as well as in the extended states were formed in case of excess nitrogen supply rate. The structural defects that were caused by the suppression of the surface migration of group-III atoms with excessive activated nitrogen might have worked as non-radiative recombination centers. Thus, a slightly N-rich condition is desirable for the realization of the uniform InGaN alloy composition and the high optical quality, especially at relatively high growth temperature.

This paper presents the experimental observation of the degradation in asymmetric sampled grating DFB lasers by the accelerated life tests. Two degradation phenomena related to the electrical characteristics of LDs are observed during the tests. The first degradation phenomenon by increasing the reverse current is considered as a formation of leakage current path enough to prevent lasing operation in lateral blocking layer near active region of lasers. The second degradation phenomenon by decreasing the forward current is considered as activation of non-radiative Auger recombination process by thermal energy. It is also experimentally observed that the second degradation phenomenon is recovered after remained in room temperature with no electrical stress. Therefore, the criteria for LD reliability can be determined by observing the degradation of the reverse current-voltage characteristics.

Polycrystalline Cu(In,Ga)Se 2 films (CIGSe) show substantial local variations of properties not only in regime of 10-100 nm but also in the scale length of few microns. We have analyzed optoelectronic properties of CIGSe heterodiodes by confocal luminescence and focused light beam induced currents (LBIC) versus temperature and excitation level with < 1 μm lateral resolution and we observe a strong dependence of the size of local patterns on excitation flux and a considerable dependence of the yield and the spectral shape of luminescence and of microscopic LBI currents on temperature. From experiments we derive activation energies for rates of non-radiative recombination of (2-7) meV and for minority carrier mobilities of about (60-70) meV. These energies are compared with local variations of band edges resulting from potential fluctuations which are formulated after an approach from literature and which has been fitted to experimental shifts of PL peaks and squeezing of PL spectra versus excitation flux. We estimate tunnel barriers for radiative transitions of trapped electrons of about (30-70) meV. Correlating our different results we attribute the activation energy for minority transport in CIGSe reflecting local variations of the conduction band edge mainly to spatial fluctuations of the optical band gap as a consequence of spatially varying elemental composition, and to variations of splitting of the quasi-Fermi levels introduced by spatially varying defect densities.

A principal origin of the defect-insensitive emission probability of In-containing three-dimensional (3-D) bulk (Al,In,Ga)N alloy films, such as InGaN, AlInN and AlInGaN, is proposed. In contrast to In-free GaN or AlGaN, In-containing films grown on sapphire substrates show significant emission probabilities, although the threading dislocation density generated due to lattice mismatch is six orders of magnitude higher than that in conventional (Al,In,Ga)(As,P) light-emitting-diode films. According to the extremely short diffusion lengths of positrons (

We present a study of the visible luminescence in ZnO hydrothermal crystals. The study is carried out on oxygen polarity basal plane (000-1) plates that are sliced through several distinct growth sectors of the crystal. The plates reveal three distinct regions that display differences in the luminescence emission, especially in the visible range. These differences should be related to the variation in the distribution of impurities and native defects in the various growth sectors of hydrothermal ZnO. CL images also expose the presence of nonradiative recombination centers that are associated with point defects or complexes of point defects. CL measurements varying the acceleration voltage of the e-beam reveal that the deep levels responsible for the green luminescence, and in a minor extension those responsible for the yellow-orange luminescence are depleted near the surface. A discussion about the main hypothesis accounting for such behavior is presented.

The temperature, excitation power and polarization dependence of ultraviolet (UV) photoluminescence (PL) of InGaN/AlGaN light-emitting diodes (LEDs) grown on (0001) and (1120) sapphire substrates were investigated. It appears that the LEDs grown on (1120) sapphire substrates show higher integrated luminescent efficiency than that of the LEDs grown on (0001) sapphire substrates. From the experimental data, it is believed that, for the InGaN LEDs having reduced InN molar fraction in the InGaN well layer, the PL characteristics are determined by the competition between the QW (or QD) radiative recombination, spatially localized radiative recombination and defect-induced nonradiative recombination. According to the results of polarization dependent edge-emitting PL measurements, the LEDs grown on (1120) sapphire substrates were found to exhibit a QW-like behaviour, while the LEDs grown on (0001) sapphire substrates were observed to show a mixed QW/QD-like behaviour. The polarization dependent edge emitting PL measurement is considered to be a highly sensitive technique for the characterization of the nanostructures of InGaN MQW LEDs.

Self-assembled InAs bi-quantum-dot molecules (BQDMs) on GaAs (0 0 1) have been achieved using gas-source molecular beam epitaxy (GSMBE) and characterized by atomic force microscopy (AFM) and photoluminescence (PL) measurements. After the initial InAs quantum dots are partially capped with GaAs and followed by deposition of InAs, BQDMs are formed. Photoluminescence spectra of BQDMs can be described by two Gaussian-fitted curves at low temperature due to two different dot sizes. This is consistent with dot height histograms obtained from AFM images of BQDMs. For PL intensity, thermal quenching is observed at temperatures above 100 K. The activation energy of BQDM PL corresponds to nonradiative recombination due to defected-related states and potential fluctuation.

Temporal aspects of recombination and energy transfer processes in Er-doped SiO 2 sensitized with Si nanocrystals (Si-nc's) were investigated by luminescence and excitation spectroscopy using time-correlated photon counting. This detection mode allows that emissions of very different intensities and dynamics may be investigated simultaneously, without loss of time resolution or amplitude deformation. In this way, components with decay constants ranging from nano- to milliseconds were identified in the luminescence bands of Si-nc's, Er 3+ ions, and defects. We postulate to relate these to recombination processes originating from isolated Er 3+ ions and Er 3+ ions located inside or in direct vicinity of Si-nc's, with dynamics in the milli- and microsecond, and nanosecond range, respectively. In this way, a unique picture of the mutual relation between the two subsystems of Er 3+ ions and Si-nc's, and truly microscopic information on the sensitization effect is obtained. Based on this new information, we conclude on a strong enhancement of non-radiative recombination of Er 3+ upon sensitization with Si-nc's and put forward a complete description of Si-nc's as sensitizers of SiO 2 :Er system, where all the Er 3+ ions available in the system are accounted for.

MgSe/BeZnSeTe II–VI compound superlattices (SLs) were grown on InP substrates by molecular beam epitaxy (MBE). In the growth, Zn molecular beam irradiation was performed at each hetero-interface between MgSe and BeZnSeTe layers to suppress in-plane compositional fluctuation of the BeZnSeTe layers and to enhance steepness of the interfaces. The Zn irradiation effect was systematically investigated changing the Zn irradiation time. As a result, it was found that structural and optical properties of the SLs strongly depended on the Zn irradiation time. Explicit high-order X-ray diffraction (XRD) satellite peaks appeared by introducing the Zn irradiation, which means the interface steepness was improved. In photoluminescence (PL) measurements at room temperature (RT), single peak emissions around 520 nm were observed. PL intensities at 15 K and RT strongly depended on the Zn irradiation time, and they were maximized when the Zn irradiation time was 4 s. The supply amount of the Zn irradiation for 4 s corresponds to about two-monolayer growth. From the dependencies of the PL intensity, it was shown that appropriate Zn irradiation was effective for both increase in the radiative recombination rate and decrease in the non-radiative recombination rate.

We have investigated the dependence of lifetime distribution of photoluminescence (PL) in a-Si:H films on the defect density. In order to analyse the lifetime distribution quantitatively, we estimated characteristic lifetime from the in-phase signal obtained by frequency resolved spectroscopy (FRS). The increase of lifetime of PL due to contribution of emission from triplet excitons trapped at defects is seen at 0.95–1.13 eV. The characteristic lifetime of emission from electron-hole pairs is discussed by considering the probability of tunnelling to non-radiative recombination centres and radii of wave functions of gap states and tail states.

The metrological capability of the picosecond four-wave mixing (FWM) technique for evaluation of the photoelectrical properties of GaN heterostructures grown on sapphire, silicon carbide, and silicon substrates as well as of free-standing GaN films is demonstrated. Carrier recombination and transport features have been studied in a wide excitation, temperature, and dislocation density (from ∼10 10 to 10 6 cm -2 ) range, exploring non-resonant refractive index modulation by a free carrier plasma. The studies allowed to establish the correlations between the dislocation density and the carrier lifetime, diffusion length, and stimulated emission threshold, to reveal a competition between the bimolecular and nonradiative recombination, and to verify the temperature dependence of bimolecular recombination coefficient in the 10-300 K temperature range. It was shown that the FWM technique is more advantageous than the time-resolved photoluminescence technique for determination of carrier lifetimes in high quality thick III-nitride layers.

High quality In 0.13 Ga 0.87 N/GaN multiple quantum wells (MQWs) on (0001) sapphire substrate were fabricated by MOCVD method. The quantum well thickness is as thin as 10 A, and the barrier thickness is 50 A. We have investigated these ultrathin MQWs by continuous wave (cw) and time-resolved spectroscopy in the picosecond time scales in a wide temperature range from 10 to 290 K. In the luminescence spectrum at 10 K, we observed a broad peak at 3.134 eV which was attributed to the quantum wells emission of InGaN. The full width at half maximum of this peak was 129 meV at 10 K and the broadening at low temperatures which was mostly inhomogeneous was thought to be due to compositional fluctuations and interfacial disorder in the alloy. We also observed an intense and narrow peak at 3.471 eV due to the GaN barrier. The temperature dependence of the luminescence was studied and the peak positions and the intensities of the different peaks were obtained. The activation energy of the InGaN quantum well emission peak was estimated as 69 meV. From the measurements of luminescence intensities and lifetimes at various temperatures, radiative and non-radiative recombination lifetimes were deduced. The results were explained by considering only the localization of the excitons due to potential fluctuations.

We present the results based upon a systematic study of the properties of quantum dot (QD) lasers with emission wavelengths around 0.98 and 1.3 μm at room temperature and atmospheric pressure. To investigate the radiative and non-radiative components of the threshold current, we studied the temperature and high hydrostatic pressure dependencies of spontaneous and stimulated emission. Although important parameters such as lasing wavelength, QD density, ridge width, cavity length, threshold current density (J th ) varied greatly, we found that all the lasers have nearly the same dependence of the radiative component, J rad , on band gap when it was tuned by the application of high pressure. It was observed that J rad increases strongly with band gap. Therefore the different dependencies of J th are explained in terms of the relative importance of different non-radiative recombination mechanisms, such as Auger recombination and thermal carrier escape.

(GaIn)(NAs)/GaAs multi-quantum-well heterostructures are grown by metal organic vapor phase epitaxy at low temperatures and are subsequently annealed in the reactor to optimize optoelectronic properties. Detailed optical and structural studies of the properties of the material, which change upon annealing under different As-stabilization as well as at different temperatures, reveal that there are two major effects of the anneal. The first one is the blue-shift of the fundamental band gap of the material, which can be attributed to a local change in the group-III environment of the nitrogen atoms. The second observation is a strong increase in photoluminescence (PL) intensity and decrease in PL linewidth upon anneal in H2-ambient, which can be attributed to the removal of non-radiative defects in the material. Chain-like N-ordering in growth direction, which induces strong inhomogeneous strain fields in the material and which can be dissolved upon anneal in H2-ambient might also act as one of the main non-radiative recombination centers in the (GaIn)(NAs) material system.

We have studied the pressure and temperature dependence of the threshold current density, J th , in 1.3 μm and 1.5 μm GaInNAs edge-emitting lasers. We find that J th is more temperature sensitive for the 1.5 μin devices. From analysis of the spontaneous emission from these devices we find that J th for the 1.3 μm devices is comprised of 50% non-radiative monomolecular recombination, 25% radiative recombination and 25% non-radiative Auger recombination at room temperature. In the 1.5 μm devices however it is composed of 30% monomolecular recombination, 10% radiative recombination and 60% Auger recombination. It is clear that the major difference in J th between the devices is caused by the increased Auger recombination in the 1.5 μm devices. Previously, we found that J th for 1.3 μm GaInNAs lasers increases with pressure due to the increase in Auger recombination brought about by a large increase in the cubed threshold carrier density (n 3 th ). However, for the 1.5 μm devices, we find no significant change in J th with pressures up to 8 kbar. We suggest that this is due to the higher Auger coefficient for the 1.5 μm devices, which compensates for the increase in n 3 th with pressure.

We investigated the temperature and pressure dependence of carrier recombination processes occurring in GaAsSb edge-emitting lasers operating near 1.3 μm. Below ∼100 K, the threshold current, I th , is dominated by the radiative current, I rad , and is proportional to temperature, T. However, above 100 K, non-radiative recombination increases abruptly such that by 125 K it accounts for 40% of I th . From high pressure measurements at this temperature, we find that the non-radiative current decreases with increasing pressure, consistent with the presence of Auger recombination. At room temperature, non-radiative recombination accounts for ∼90% 4 and gives rise to a super-linear temperature dependence of I th , in spite of the fact that I th ∝ T. At room temperature the non-radiative current increases with increasing pressure, indicating that under ambient operating conditions, the devices are also limited by carrier leakage into the r-minimum of the GaAs barriers and possibly also into the X-minima of the GaAsP confining layers.

Organic semiconductors typically possess low charge carrier mobilities and Langevin-type recombination dynamics, which both negatively impact the performance of organic solar cells and photodetectors. Charge transport in organic solar cells is usually characterized by the mobility-lifetime product. Using newly developed transient and steady state photocurrent measurement techniques we show that the onset of efficiency limiting photocarrier recombination is determined by the charge that can be stored on the electrodes of the device. It is shown that significant photocarrier recombination can be avoided when the total charge inside the device, defined by the trapped, doping-induced and mobile charge carriers, is less than the electrode charge. Based upon this physics we propose the mobility-recombination coefficient product as an alternative and more convenient figure of merit to minimize the recombination losses. We validate the results in 3 different organic semiconductor-based light harvesting systems with very different charge transport properties. The findings allow the determination of the charge collection efficiency in fully operational devices. In turn, knowing the conditions under which non-geminate recombination is eliminated enables one to quantify the generation efficiency of free charge carriers. The results are relevant to a wide range of light harvesting systems, particularly those based upon disordered semiconductors, and require a rethink of the critical parameters for charge transport.

Blue InGaN LEDs have a drawback whereby it suffers from efficiency droop at high current injection levels. One of the major factors of this droop behaviour is contributed by electron leakage. The aim of this study was to suppress electron leakage in the device by understanding how barrier thickness affects the carrier distribution in the device as well as the device performances. Simulation results obtained showed that thinner barrier increased the device’s efficiency since there was better electron confinement and tunnelling effect.

We reveal substantial luminescence yield heterogeneity among individual subdiffraction grains of high-performing methylammonium lead halide perovskite films by using high-resolution cathodoluminescence microscopy. Using considerably lower accelerating voltages than is conventional in scanning electron microscopy, we image the electron beam-induced luminescence of the films and statistically characterize the depth-dependent role of defects that promote nonradiative recombination losses. The highest variability in the luminescence intensity is observed at the exposed grain surfaces, which we attribute to surface defects. By probing deeper into the film, it appears that bulk defects are more homogeneously distributed. By identifying the origin and variability of a surface-specific loss mechanism that deleteriously impacts device efficiency, we suggest that producing films homogeneously composed of the highest-luminescence grains found in this study could result in a dramatic improvement of overall device efficiency. We also show that although cathodoluminescence microscopy is generally used only to image inorganic materials it can be a powerful tool to investigate radiative and nonradiative charge carrier recombination on the nanoscale in organic-inorganic hybrid materials.

Giant 60-fold enhanced ultraviolet (UV) emission is obtained in Pt nanoparticle-assembled ZnO film. Besides surface plasmons coupling, the conversion of non-radiative recombination into UV emission makes great contributions to the enhancement. It paves a new way in designing high-efficiency UV optoelectronic devices without defect-related energy loss.

Self-assembled In(Ga)As/GaAs quantum dot infrared photodetectors (QDIPs) have promising applications in the midwavelength infrared and long-wavelength infrared regions for various defense and space application purposes. It has been demonstrated that the performance of QDIPs has improved significantly by using architectures such as dots-in-awell, different combinational capping or post growth treatment with high energy hydrogen ions. In this work, we enhanced the electrical properties InGaAs/GaAs using high energy proton implantation. Irradiation with proton resulted suppression in field assisted tunnelling of dark current by three orders for implanted devices. Photoluminescence (PL) enhancement was observed up to certain dose of protons due to eradication of as-grown defects and non radiative recombination centers. In addition, peak detectivity (D*) increased up to two orders of magnitude from 6.1 x108 to 1.0 × 1010 cm-Hz1/2/W for all implanted devices.

We developed a straightforward method based on detailed balance relations to analyze individual subcells in multi-junction solar cells via measuring absolute electroluminescence quantum yields. This method was applied to characterization of a InGaP/GaAs/Ge 3-junction solar cell for satellite use. In addition to subcell I-V characteristics and internal luminescence yields, we derived balance sheets of energy and carriers, which revealed respective subcell contributions of radiative and nonradiative recombination losses, junction loss, and luminescence coupling. These results provide important diagnosis and feedback to fabrications. We calculated conversion-efficiency limit and optimized bandgap energy in 2-, 3-, and 4-junction tandem solar cells, including finite values of sub-cell internal luminescence quantum yields to account for realistic material qualities in sub-cells. With reference to the measured internal luminescence quantum yields, the theoretical results provide realistic targets of efficiency limits and improved design principles of practical tandem solar cells.